The United States Senate will take a gigantic leap into the unknown today, formally beginning the impeachment trial of President Bill Clinton on charges of perjury and obstruction of justice without any form of agreement on the procedure that will follow.

The lack of agreement on the scope of the process between the Senate's 55 Republicans and 45 Democrats - as well as the divisions within the Republican majority itself - leaves open the possibility that the Clinton trial will now last far longer and become more destructive than most political observers have recently acknowledged.

The swearing-in of Chief Justice William Rehnquist to preside over the trial, and of the 100 senators to act as jurors, will take place without the participants having a clear idea of what is to follow.

In Washington yesterday, the Senate majority leader, Trent Lott, told the reconvening Senate that the trial could last for three weeks or 'longer than that' and he expected to have details 'within 24 hours'.

His inability to give such details reflected another day of hard bargaining behind the scenes and has raised doubts about his ability to steer the impeachment trial through the Senate.

The signs are that his emerging plan will fail to satisfy either the president's defenders or his accusers.

Mr Lott has come under intense pressure from conservative Republicans in both houses of Congress to allow a lengthier trial than originally envisaged, involving the calling of witnesses - including the former White House intern Monica Lewinsky.

The House 'managers' or prosecutors, led by Congress-man Henry Hyde, lobbied Mr Lott yesterday for witnesses to testify during the trial, a move which angered Democrats and brought a whiff of the House anti-Clinton partisanship which Mr Lott and the Senate Republicans have been striving to avoid.

The White House's legal team made it clear that in such a case it would mount an exhaustive defence of the president, calling witnesses such as Linda Tripp and Lucianne Goldberg, extensively cross-examining Ms Lewinsky, and using all available procedural tactics to guarantee a lengthy trial.

The re-emergence of such a prospect follows intense conservative criticism of the bipartisan scheme drafted by Senator Joseph Lieberman, a Democrat, and Senator Slade Gorton, a Republican, which would have led to an early 'test vote' next Thursday to discover whether there was a Senate majority for continuing the trial.

Mr Lott, facing a choice between leading a minority of his party into a compromise or consolidating his position with his own rank-and-file, has chosen the second course, making an early end to the crisis look less likely by the day.

Jury ServiceBy Julian Borger

What will happen on the first day of the Senate trial today?

The Senate will send one of its officers across the Capitol building to notify the House of Representatives that it is prepared to receive the two articles of impeachment, for perjury and obstruction of justice.

Thirteen House 'managers' (in effect, prosecutors) are led to the Senate by the judiciary committee chairman, Henry Hyde, who will read the impeachment charges.

The Senate will notify the US chief justice, William Rehnquist, that his presence is required to preside over the proceedings.

Mr Rehnquist will be sworn in as presiding officer by another member of the supreme court. He will have authority to rule on the admissibility of evidence and witnesses, but can be overruled by a majority of the 100 Senators.

He will administer an oath to the senators, asking them to swear: 'I will do impartial justice according to the constitution and laws, so help me God.' The trial will open, almost certainly with discussion of when to reconvene. That will probably be on Monday.